Field of the Invention
The present invention relates to an equivalent phantom and a method of evaluating the quality of the X-ray Talbot imaging apparatuses with the equivalent phantom.
Description of Related Art
An X-ray imaging apparatus is known, which is provided with a Talbot interferometer or a Talbot-Lau interferometer and an X-ray detector (Flat Panel Detector: FPD) and captures and visualizes phase shift of X-rays passing through an object (see, for example, Japanese Unexamined Patent Application Publication No. 2008-200359; WO2011/033798; K. Hibino et al, J. Opt. Soc. Am. A, Vol. 12, (1995) p. 761-768; A. Momose et al, J. Appl. Phys., Vol. 45, (2006) p. 5254-5262; and M. Takeda et al, J. Opt. Soc. Am, Vol. 72, No. 1, (1982) p. 156). The X-ray imaging apparatus provided with such a Talbot or Talbot-Lau interferometer is hereinafter referred to as X-ray Talbot imaging apparatus.
The X-ray Talbot imaging apparatus includes a first grating (also known as a G1 grating) and a second grating (also known as a G2 grating), each consisting of slits at constant intervals, and an optional X-ray source grating, if a Talbot-Lau interferometer is included. An X-ray source emits X-rays to the first grating to produce an image of the first grating itself at a focused position downstream of the first grating in the direction of traveling X-rays. The second grating is disposed at this position in such a manner that the array of slits of the second grating is slightly slanted to the array of the slits of the first grating. This slanted disposition produces Moire fringes onto the second grating. These Moire fringes are superimposed to produce an image (Moire image, hereinafter). The image is detected and visualized by the X-ray detector disposed downstream of the second grating.
An object placed between the X-ray source and the first grating distorts the Moire fringes. The X-ray Talbot imaging apparatus moves the first and second gratings relatively to each other to capture plural Moire images (Fringe scanning mode). The Moire images are analyzed through image processing to reconstruct images such as a differential phase image, an X-ray absorption image, and a small angle scattering image. Alternatively, a Moire image of an object captured with the X-ray Talbot imaging apparatus is processed through, for example, Fourier transformation to reconstruct an image such as a differential phase image (Fourier transformation method).
Conventional images such as an X-ray absorption image (see FIG. 8A) have not been able to include a cartilage end (exactly, an “interface” between a cartilage and surrounding joint fluid in a joint, hereinafter). The present inventers have found that a differential phase image reconstructed from the Moire image as described above can include a cartilage end as shown with an arrow in FIG. 8B. The study by the inventors also shows that the differential phase image can include not only joint cartilages but also, for example, human soft tissues such as Achilles tendons and tumor masses.
Such a differential phase image reconstructed from the Moire image captured with the X-ray Talbot imaging apparatus can include a joint cartilage end, only if each grating such as the first grating is precisely manufactured (or, within an allowable manufacturing tolerance), and if the X-ray Talbot imaging apparatus is well evaluated and controlled in quality such that MTFs (Modulation Transfer Function) of a X-ray source tube and the X-ray detector are in good conditions. If the quality is not properly evaluated or controlled, the differential phase image cannot include soft tissues such as a joint cartilage.
The evaluation and control of quality as well as improvement in quality have been performed for individual components of the X-ray Talbot imaging apparatus. For example, an X-ray source tube has been evaluated through imaging of a test pattern to determine the diameter of a focal spot; an X-ray detector (FPD) through imaging of its profile and edges with X-rays to determine the MTF; or the manufacturing accuracy of the grating through comparison of Moire images at the start and the end of the operation of an X-ray Talbot imaging apparatus to check for any distortion in relative positions to adjust individual components, as described in WO 2008/102685.
Even if each component were evaluated and/or adjusted, the component of the X-ray Talbot imaging apparatus would deteriorate in quality over time. Therefore, it is not always ensured that a differential phase image reconstructed from the Moire images captured with the X-ray Talbot imaging apparatus includes soft tissues of joint cartilages. In other words, to ensure capture of soft tissues of joint cartilages in a differential phase image reconstructed from the Moire images captured with the X-ray Talbot imaging apparatus, the quality of the X-ray Talbot imaging apparatus itself, that is, the overall quality of the X-ray Talbot imaging apparatus should be evaluated.
The X-ray Talbot imaging apparatus, however, is still in the development phase and has no criteria required for evaluation. If the X-ray Talbot imaging apparatus is regarded as an imaging apparatus to draw an image of a soft tissue in a differential phase image, it is preferable in imaging an object that the quality of the X-ray Talbot imaging apparatus is defined as the image quality of the differential phase image reconstructed from the Moire image captured with the X-ray Talbot imaging apparatus.
If the image quality is defined such that the image quality of human soft tissues can be estimated by imaging the equivalent phantom with the X-ray Talbot imaging apparatus, variations in quality between apparatuses and days can be controlled. In addition, different types of imaging apparatuses also can be compared in quality, which is practically advantageous.